Cell growth is controlled, to a large degree, by extracellular ligands which bind to specific receptors on the surface of cells. Cross et al., Cell , 64, 2172 (1991). A number of these receptors, including the epidermal growth factor (EGF) receptor, have intrinsic protein tyrosine kinase (PTK) activity. Yarden et al., Ann. Rev. Biochem., 57, 443 (1988). Ligand-dependent activation of receptor associated tyrosine kinases or unregulated synthesis of tyrosine kinase oncoproteins results in tyrosine phosphorylation of cellular substrates which have a critical role in the control of mitogenesis, cell cycle regulation, cell survival and cellular transformation. Ullrich et al., Cell, 61, 203 (1990).
Among the cellular enzymes that are involved in signal transduction, the protein tyrosine kinases (PTKs) appear to play key roles in the initiation of various signaling cascades. PTKs can be divided into two major groups on the basis of their predicted structures. The first PTK group, which contains those that possess extracellular domains which generally function to bind peptide hormones, are the receptor PTKs. Examples of PTKs included in this group are the receptors for epidermal growth factor, the nerve growth factor and platelet-derived growth factor.
The second PTK group comprises those that lack the extracellular domains and are referred to as nonreceptor PTKs, even though many members of this group appear to be associated, albeit noncovalently, with some type of cell surface ligand-binding protein. Members of this group include the Src family of PTKs as well as the members of the fes/fps and abl gene families. The nonreceptor class of PTKs is growing with regard to the number of enzymes it includes, which are also demonstrating surprising diversity in predicted structure.
The Src family of nonreceptor PTK enzymes currently contains nine members: Src, Yes, Fyn, Lyn, Lck, Hck, Fgr, Blk, and Yrk. The Src, Yes, Fyn, and Lyn proteins are expressed in a variety of cell types, whereas the Lck, Hck, Fgr and Blk proteins are expressed primarily in different types of hematopoietic cells. Also, Src is expressed by the cells associated with colon cancer, breast cancer and ovarian cancer as well as virtually all other forms of human cancer. Likewise, Fyn and Lyn are expressed in virtually all forms of human cancer.
Tyrosine-specific protein kinase activity is also known to be associated with oncogene products of the retroviral Src gene family. Hunter et al., Annu. Rev. Biochem., 54, 897 (1985). This kinase activity is strongly correlated with the ability of retroviruses to transform cells, since mutants with reduced kinase activity have lower transforming efficiency, and mutants which lack tyrosine kinase activity are transformation defective. Bishop, Annu. Rev. Biochem., 52, 301 (1983). Similar kinase activity is also associated with the cellular receptors for several growth factors such as EGF, platelet derived growth factor, insulin, and insulin-like growth factor I. Ushiro et al., J. Biol. Chem., 255, 8363 (1980); Ek et al., Nature, 295, 419 (1982); Kasuga et al., Nature, 298, 667 (1982); Jacobs et al., J. Biol. Chem., 258, 9581 (1983). Therefore, it is likely that tyrosine phosphorylation plays an important role for cell proliferation and malignant cell transformation, and a drug capable of PTK inhibition would be likely to exhibit desirable antiproliferative, pro-differentiating effects. Therefore, PTKs represent potential targets for the development of anti-cancer drugs or drugs intended to control pathologies associated with abnormal cellular proliferation.
A number of PTK inhibitors have been investigated as potential anticancer reagents. They include isoflavones (genistein), tyrphostins (erbstatin), lavendustin analogues, staurosporine analogues (dianilinophthalmides), polyhydroxylated stilbene analogues of piceatannol, dithiobis (indole-alkanoic acid), dihydroxyisoquinolines and others. For example, see T.R. Burke, Jr., "Protein-tyrosine kinase inhibitors," Drugs of the Future, 17, 119 (1992); P. Workman et al., Seminars in Cancer Biology, 3, 369 (1992); and D.W. Fry, Exp. Opin. Invest. Drug, 3, 577 (1994).
For the purpose of obtaining highly specific inhibitors, bicyclic compounds as ring-constrained inhibitors of PTK have recently been introduced. They are expected to interact with the flat, cleft-like catalytic cavity of the kinase domain with high specificity. Iminochromenes belong to this type of compound. For example, several 3-carbamoyl-2-iminochromenes with weak PTK inhibitory activity toward p56.sup.lck have been reported by T.R. Burke et at., J. Med. Chem., 36, 425 (1993). These compounds are of general formula (1): ##STR3## wherein X is 6-, 7- or 8-hydroxy or 6,7 or 7,8-dihydroxy. Earlier, C. N. O'Callaghan et al., Proc. R.I.A., 79, 87 (1979) reported that compounds of formula (1) wherein X is 6- or 8-methoxy, 6-chloro, 6-nitro or 8-ethoxy exhibited anti-tumor activity against P388 murine lymphocytic leukemia. G. Keri et al. (published PCT application WO 93/16084) disclose compounds of formula (1) wherein X represents up to four benzo-substituents, including trihydroxy or trialkoxy. These compounds are generally disclosed to be anti-tumor agents due to their ability to inhibit TK enzymes. However, no biological data was reported for these compounds.
Therefore, a need exists for selective TK inhibitors which broadly inhibit the pathological division of cells, such as tumor cells, without exhibiting undesirable cytotoxicity to normal cells.